A fuel cell is an effective device that can convert chemical energy into electrical energy through electro-catalytic reactions. A proton exchange membrane fuel cell (hereinafter, “PEMFC”.) operates at a relatively low temperature with a gas phase hydrogen provided as fuel and oxygen (air) as an oxidant. Due to its high conversion efficiency, low noise and low emissions, PEMFC has high potential for many uses in automobile applications and distributed power generation.
At the core of a PEMFC is the membrane electrode assembly (hereinafter, “MEA”) which includes an anode, a cathode and a polymer electrolyte layer in-between. At the surface of the anode, hydrogen is oxidized to a proton started through the electro-catalytic process,H2→2H+2e−  (1)
The protons thus produced are transported to the cathode side through a proton conductive membrane. At the surface of the cathode, oxygen is electro-catalytically reduced and subsequently reacts with protons from the equation (1) to form water,O2+4e−+4H+→2H2O  (2)
The reaction (2) is also known as the oxygen reduction reaction (hereinafter referred to as “ORR”). The reactions (1) and (2) occur on the surface of the electrode catalysts. At present, the most effective catalysts for these reactions are made of platinum supported on amorphous carbon. A typical Pt loading on the MEA surface ranges from 0.2 mg/cm2 to 0.4 mg/cm2. Since platinum is a precious metal with limited supply, its usage adds a significant cost to a PEMFC system.
Consequently, there is a substantial need for replacement materials for the catalyst to reduce costs and insure adequate material supplies for wide scale use in fuel cells, as well as other applications. Few catalyst metals have been found to have a comparable catalytic efficiency as that of platinum for the ORR. Those catalysts found with similar catalytic activity usually belong to the precious group metals (hereinafter referred to as “PGM”), such as Pd, Rh, Ir, Ru and others, in addition to Pt. The PGMs generally are very costly due to limited reserves worldwide. As noted hereinbefore, the use of PGMs for an electrochemical device, such as fuel cells, will add significant cost to the system, therefore, creating major barriers for commercialization.
There have been many attempts to replace PGMs, mainly through use of the transition metal compounds. For example, it has been known that the molecules containing a macrocyclic structure with an iron or cobalt ion coordinated by nitrogen from the four surrounding pyrrolic rings have the catalytic activity to capture and to reduce molecular oxygen. It has been demonstrated that ORR catalytic activity can be further improved for such systems containing coordinated FeN4 and CoN4 macrocycles if they have been heat-treated. Examples of macro-molecular system containing FeN4 and CoN4 moieties include corresponding transitional metal phthalocyanine and porphyrin. Recent experiments have shown a similar method of making amorphous carbon based catalyst with good ORR activity by mixing macromolecules with FeN4 group and carbonaceous material or synthetic carbon support, followed by high temperature treatment in the gas mixture of ammonia, hydrogen and argon. Alternative study also found that high temperature treatment of iron salt deposited on the carbon in the presence nitrogen precursor can also produce catalyst with very good ORR activity. The catalytic activity is attributed to the active site with a phenanthroline type structure where Fe ion is coordinated to four pyridinic nitrogens. It was also found that the catalyst thus produced decomposed in an acidic condition to release iron, and thus is unstable for the electro-catalytic reaction such as for inside a fuel cell cathode. Recently, an issued U.S. patent discussed a method of preparing non-PGM catalyst by incorporating transition metal to heteroatomic polymers in the polymer/carbon composite. In addition, this patent further discussed a method to improve the activity of polymer/carbon composite by heat-treating the composite at elevated temperature in the inert atmosphere of nitrogen. Nevertheless, none of these methods or articles of manufacture have resulted in an adequate solution to the above stated problem of replacement materials of reasonable cost and adequate supplies for large scale use in fuel cells, as well as other applications requiring such catalysts.